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Weather 101 - Cloud Formations

You wouldn’t leave the dock without first checking the local weather forecast. You can get weather information from TV, radio, your VHF radio and on the Internet (see Safety Links above). While on the water, your VHF radio is the best source for weather warnings. Even so, at certain times of the year weather can change rapidly and you should continually keep a "weather eye" out, especially to the west, in order to foresee changes which might be impending.

Clouds are a tool you can use to predict or forecast weather. The type of cloud and direction of movement can warn you of weather changes that are imminent. Clouds are categorized by the altitude at which they appear and the shape that they take.

(This is not an in-depth study of clouds, but an attempt to simplify the subject for use by recreational boaters.)

Cloud Group Cloud Height Cloud Types
High Clouds = Cirrus Above 18,000 feet Cirrus
Cirrostratus
Cirrocumulus
Middle Clouds = Alto 6,500 feet to 18,000 feet Altostratus
Altocumulus
Low Clouds = Stratus Up to 6,500 feet Stratus
Stratocumulus
Nimbostratus
Clouds with vertical growth   Cumulus
Cumulonimbus

It is helpful to remember the following definitions of cloud shapes:

cumulus5.jpg (2658 bytes) Cumulus meaning "heap, a pile, an accumulation"
stratus5.jpg (3159 bytes) Stratus meaning "spread out, flatten, cover with a layer"
stormcloud2.jpg (2474 bytes) Nimbus meaning "rainy cloud"

Variations of cloud types are created by combining the cloud’s shape/description with the altitudinal names as a prefix or suffix.

Cirros (high) or Cirro can be used with cumulus (heap) to indicate a cirrocumulus or high, lumpy cloud. Cirrocumulus clouds, sometime called "mackerel skies", can indicate the approach of a hurricane in the tropics. It can also be used with stratus (flat, layered)as in cirrostratus to indicate a high, flat or layered cloud.

Alto can also be used with cumulus and stratus to indicate altocumulus and altostratus which are middle altitude lumpy clouds and middle altitude layered clouds respectively.

Nimbo or nimbus might be used with cumulus or stratus to indicate a cloud formation that is producing precipitation. These clouds could be either cumulonimbus which would be a lumpy, vertically-rising rain cloud or nimbostratus which would be a sheet or flat-looking rain cloud.

High clouds exist above 18,000 feet and are cirrus clouds.

cirrus.jpg (2106 bytes) Cirrus clouds are the most common of the high clouds. They are composed of ice and consist of long, thin, wispy streamers. Cirrus clouds are usually white and predict fair weather. Sometimes called mares tails, they stream with the wind. By watching the movement of cirrus clouds you can tell from which direction weather is approaching. The appearance of cirrus clouds usually indicates that a change in weather will occur within 24 hours.
cirrostratus.jpg (2244 bytes) Cirrostratus are sheetlike, thin clouds that usually cover the entire sky. The sun or moon can shine through Cirrostratus clouds. Cirrostratus clouds usually come 12-24 hours before a rain or snow storm.
cirrocumulous.jpg (1969 bytes) Cirrocumulus are small, rounded puffs that usually appear in long rows. They are usually white, but sometimes appear gray. Cirrocumulus are usually seen in the winter and indicate fair, but cold, weather. In the tropics, they may indicate an approaching hurricane.

Medium high clouds occupy altitudes of 6,500 feet to 18,000 feet. These clouds are called alto clouds. Alto clouds are used to predict weather changes in 6 to 12 hours.

altostratus9.jpg (2660 bytes) An Altostratus cloud usually covers the whole sky. The cloud looks gray or blue-gray. The sun or moon may shine through an Altostratus cloud, but will appear hazy. An altostratus cloud usually forms ahead of storms with continuous rain or snow.
altocumulus.jpg (2601 bytes) Altocumulus clouds are grayish-white with one part of the cloud darker than the other. Altocumulus clouds usually form in groups. If you see Altocumulus clouds on a warm, sticky morning, be prepared for thunderstorms by late afternoon.

Low clouds, called stratus clouds, are at altitudes up to 6,500 feet. These clouds form a solid sheet or layer of cloud mass.

stratus2.jpg (3278 bytes) Stratus clouds are uniform gray in color and almost cover the entire sky. Light mist or drizzle is sometimes associated with Stratus clouds.
stratocumulus.jpg (3331 bytes) Stratocumulus clouds are low, lumpy and gray. Most form in rows with blue sky visible in between. Precipitation rarely occurs with Stratocumulus clouds, however, in frontal weather they may turn to Nimbostratus.
nimbostratus.jpg (3293 bytes) Nimbostratus clouds are dark gray with a ragged base. Rain or snow is associated with Nimbostratus clouds.

Clouds with vertical growth

cumulus2.jpg (3420 bytes) Vertically developing clouds are the Cumulus type. These small, lumpy clouds are low "fair weather" clouds. However, as they develop vertically (by rising hot air) they may go from small, fair weather clouds to large, boiling, vertically-growing monsters called cumulonimbus.
cumulonimbus.jpg (3273 bytes) Cumulonimbus are generally known as thunderstorm clouds. High winds will flatten the top of the cloud into an anvil-like shape. Cumulonimbus are associated with heavy rain, snow, hail, lightning, and tornadoes. The anvil usually points in the direction the storm is moving.

If you still can’t remember all of the cloud names and formations, you can always watch the clouds for two specific weather situations that indicate a high probability of a storm:

  1. A "lowering ceiling": This means that the height of cloud formations continues to get lower and lower, usually caused by a warm front. As the ceiling lowers you will see clouds in the following order:
  • Cirrus
  • Cirrostratus
  • Altostratus
  • Stratus
  • Nimbostratus - storm clouds!
  1. On the other hand, watch for cumulus (puffy) clouds that start to rapidly develop vertically to become cumulonimbus thunderstorm clouds. On hot and humid days, these storms occur over water as the radiant heat from the land absorbs moisture from nearby water and rises to produce thunderheads. These storms can also indicate a cold front and may be preceded by squall lines, a row of black storm clouds. Wind shifts unpredictably and accelerates dramatically. Lightning can occur for miles in front of a storm and after the storm appears to have passed.

Other things to look for that indicate an approaching weather change:

  • Weather changes generally come from the west so scan the sky with your weather eye, especially to the west.
  • A sudden drop in temperature and change in the wind (increasing winds and/or seas) often means that a storm is near.
  • If you have a barometer on your boat check it every two to three hours. A rapid drop in pressure means a storm is approaching.

IF A STORM IS NEAR…

  • Reduce speed and proceed with caution
  • Put on PFDs.
  • Close all hatches and ports.
  • Head for the nearest shore that is safe to approach and duck into the lee of land.
  • Put the bow into the wind and take waves at about a 40-45 degree angle.
  • Watch for other boats and floating debris.
  • Pump out bilges and keep dry.
  • Change to a full fuel tank.
  • If there is lightning, unplug electrical equipment and keep away from ungrounded metal objects.
  • Secure loose items which could be tossed about.
  • Keep everyone low in the boat and near the centerline.

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BoatSafe.com